It is known that vibration is excited in a ball-striking implement as it strikes the ball and that such vibration is felt by the player in his hand and may be heard by him as sound vibration. Because it is believed that the vibration damages the hand-arm system, great attention is paid in the design of tennis rackets to the damping of such vibration. Almost all these damping measures concern the implement itself because it is in direct contact with the hand and can transmit vibration thereto. It is believed that the structural material of the implement is of high significance. For instance, it is often believed that wood is superior in damping properties than metal and glass fiber-reinforced plastics material. In the recent pertinent literature, the alleged improvements in the damping performance which are due to the use of novel fibrous materials, such as carbon fibers or boron fibers, have beem emphasized. It has also often been stated that the vibratory behavior of the racket can be influenced by the distribution of its stiffness. Besides, it has been suggested to influence the vibration by vibration-absorbing elements which are incorporated in or attached to the racket. The design of the handle as a vibration absorber is known in the art as well as the provision of spring elements connected between the strings and the hoop or between the hoop and the shaft.
It has also been stated that the vibratory behavior is influenced by the strings, and it is generally believed that in tennis rackets strings made from natural catgut have a lower tendency to vibrate than strings of plastics material. On the other hand, the choice of materials is highly restricted by the requirement for a diaphragmlike flexibility, which matches the ball. For this reason it has been virtually impossible so far to control the vibration by a change of the string material. It is also known that a variation of the initial tension of the strings influences the vibratory behavior within certain limits, and lower initial tensions are said to reduce the tendency to vibrate.
It is an object of the present invention to teach how vibration, particularly in the range above 300 Hz, can be controlled much more effectively than with the known means. This object can be accomplished in that at least two longitudinal or two transverse strings or at least one longitudinal string and one transverse string of the strings forming the striking surface are mechanically coupled to one another by means of a vibration absorber.
Vibration in that frequency range is unpleasantly felt in the hand and is audible and has been found to be generated by resonant vibration of the strings, from which the vibration is transmitted to the hoop at the points where the strings are fixed to the hoop. Whereas the resonance peak is very sharp so that the energy content of the vibration is very small, even the provision of highly effective damping means on the hoop has surprisingly failed to reduce the subjective impression of the vibration. This is due to the fact that the strings are fixed appoximately along a nodal line, and damping means attached in accordance with theory are ineffective in said nodal line. For the same reason, the players hand cannot cause the resonant vibration of the string to decay at the handle within a time which is shorter than the time threshold of perception, which amounts to about 0.1 second, although the hand has an excellent damping capacity.
On the other hand, such vibration can be absorbed in accordance with the invention in that two or more individual ones of the strings forming the striking surface are mechanically coupled to each other by a suitable coupling element. Coupling may be effected by spring and/or mass elements. The coupling elements may preferably consist of an elastomeric material which has a visocelastic inherent damping properties. The vibration of the diaphragm is damped as result of the fact that the elastomerically coupled strings are detuned relative to each other and as a result of the viscoelastic damping in the coupling element. Different coupling elements may be used which have such a spring constant and/or coupling coefficient that they cause the coupled strings to be detuned relative to each other.